Overload warning means should provide the operator with the necessary information as regards a possible overload of the device. In the case of hydraulic excavators, which can be used both in civil engineering and as a material handling device in the industry, the overload monitoring means primarily serve as safety instruments, in order to prevent the device from tilting or tipping over.
Overload warning means are already known in various configurations. The following two variants are used:
In a first variant, the hydraulic pressure in the lift cylinder is measured. During operation of the excavator, the hydraulic pressure in the lift cylinder is always monitored. By means of a payload calculation performed in advance via the configuration of the device, the lowest hydraulic cylinder pressure, at which the device still is safely standing in any case, has been determined as a reference value. This calculated pressure is adjusted at the factory by means of a pressure switch. When the pressure in the lift cylinder now exceeds the adjusted value during the load lifting operations, the operator will be warned by a corresponding alarm signal.
In a second variant, the hydraulic pressure in the lift cylinder and at the same time the boom position is measured. As already described above, the hydraulic pressure in the lift cylinder hence is monitored during operation. In addition, the boom position is, however, considered either via the angle or via the cylinder position. For the boom kinematics of the existing configuration, a payload calculation is performed in advance, in which the lowest lift cylinder pressure is calculated for each boom position. By means of these data and the characteristics of the pressure switch, a cam disk is constructed, which rotates in synchronism with the boom and adjusts the correct pressure at the pressure switch for each boom position. When the lift cylinder pressure exceeds the adjusted value during the load lifting operations, the operator will be warned by an alarm signal.
There are also used combinations of the two measuring methods. For example, in the non-supported condition of an excavator, i.e. when operating on the tires, the first variant is used, whereas for the supported condition the second variant is used (or vice versa).
In the above-described overload warning means used so far, a few problems arise, however, in practice.
For the case that the equipment position is not considered, the difference between the calculated tilting load and the actual load-carrying capacity will be up to 40%. If the boom angle now is included in the consideration, and for the remaining equipment parts the most unfavorable condition is each considered, the difference between the calculated tilting load and the actual load-carrying capacity will still be up to 20%.
If it is now desired to accurately determine the equipment position, the position must be determined for each equipment part, for instance by means of an angle detector. This in turn is time-consuming and expensive.
When the configuration of the device now is unknown or has been changed, the overload warning means no longer operates correctly, as due to the calculation from the measured data with the wrong configuration a wrong conclusion is drawn as regards the static stability.
The calculation of the loading condition only can be performed exactly for the case that the device is standing on flat ground. In the case of an inclination in longitudinal and/or transverse direction, the static moment of the machine will be reduced. In this case, the overload warning means will emit the warning signal too late.